make users-guide fails with python >= 3.13

[philderbeast]

Describe the bug
Running make users-guide fails with Python versions 3.13 and newer because the imghdr module was removed from the standard library after being deprecated in Python 3.11.

To Reproduce

$ make users-guide
...
Extension error:
Could not import extension sphinx.builders.epub3 (exception: No module named 'imghdr')
make[1]: *** [Makefile:40: ../dist-newstyle/doc/users-guide/index.html] Error 2
make[1]: Leaving directory '/home/.../cabal/doc'
make: *** [Makefile:287: users-guide] Error 2

System information

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 25.04
Release:	25.04
Codename:	plucky

[ulysses4ever]

Wow, that's sad. Are you aware of any migration guides?

[philderbeast]

I hit this after updating my Ubuntu version. I also hit an earlier problem with make users-guide-requirements:

$ make users-guide-requirements
make -C doc users-guide-requirements
make[1]: Entering directory '/home/philderbeast/dev/src/cabalism/cabal/doc'
. ../.python-sphinx-virtualenv/bin/activate \
  && pip install --upgrade pip \
  && pip install pip-tools \
  && pip-compile requirements.in
Traceback (most recent call last):
  File "/home/.../cabal/.python-sphinx-virtualenv/bin/pip", line 5, in <module>
    from pip._internal.cli.main import main
ModuleNotFoundError: No module named 'pip'
make[1]: *** [Makefile:59: requirements.txt] Error 1
make[1]: Leaving directory '/home/.../cabal/doc'
make: *** [Makefile:291: users-guide-requirements] Error 2

I'm not sure, are we meant to handle the venv setup ourselves or is the makefile meant to handle that? I got it to work with:

$ python3 -m venv .python-sphinx-virtualenv
$ make users-guide-requirements
...

I do have pip:

$ pip --version
pip 25.0 from /usr/lib/python3/dist-packages/pip (python 3.13)